CALORIMETRIC INVESTIGATION OF THE MARTENSITIC TRANSFORMATION IN NEAR STOICHIOMETRIC AND STOICHIOMETRIC Fe3Pt AND SHAPE MEMORY EFFECT

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CALORIMETRIC INVESTIGATION OF THE MARTENSITIC TRANSFORMATION IN NEAR STOICHIOMETRIC AND STOICHIOMETRIC Fe3Pt

AND SHAPE MEMORY EFFECT

N. Matveeva, G. Demensky, L. Petrov, Yu. Kovneristyi, O. Teplov

To cite this version:

N. Matveeva, G. Demensky, L. Petrov, Yu. Kovneristyi, O. Teplov. CALORIMETRIC INVES- TIGATION OF THE MARTENSITIC TRANSFORMATION IN NEAR STOICHIOMETRIC AND STOICHIOMETRIC Fe3Pt AND SHAPE MEMORY EFFECT. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-377-C4-382. �10.1051/jphyscol:1982455�. �jpa-00222171�

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CALORIMETRIC INVESTIGATION OF THE MARTENSITIC TRANSFORMATION IN NEAR STOICHIOMETRIC AND STOICHIOMETRIC Fe3Pt AND SHAPE MEMORY EFFECT

N.M. Matveeva, G.K. Demensky, L.A. Petrov, Yu.K. Kovneristyi and O.A.

Teplov

A.A. Baikov I n s t i t u t e of MetaZZurgy, Academy o f Sciences, Moscow, U.S.S.R.

(Accepted 9 A u g u s t 1982)

Abstract.

-

Heats and temperatures of the r 3 d martensitic transformation in the FegPt alloy have been measured after quen- ohing and annealing at 820 K. The results obtained are discussed in terms of heterogeneous atomic ordering A14L12 and eutectoid decomposition

x - =

^d

+

Ee3Pt, which occur during annealing and preceed the martensitic transformation.

1ntroductlon.- Alloys of the Fe-Pt system close to the Fe3Yt composition undergo a Y S o ( martensitic transformation, being of particular interest, since properties of the martensite are governed by the ot- her phase transformations, rival to the martensitic one. So the martensitic transformation of rapidly quenched alloys is not thermoelastic and is accompanied by a significant thermal hysteresis, When the martensitic transformation is preceeded by the atomic ordering

AI*L12 it becomes a thermoelastic one and has a small thermal hysteresis, The alloys are capable of recovering their shape. The effect of atomic ordering on the martensitic transformation in the iron-ba- se alloys with 23-2746 Yt has been discussed often /I-7/. Various cri- teria were proposed to determine the thermoelasticity of transformation. According to /I/ the major oriterion is given by some level of the Ms point, depending on the degree of the long-range ordering, which should necessarily be below the Curie point. This level deter- mines a small thermal hysteresis of transformation, certain tetrago- nality of the

-

^and ^o( -phases,enhancing crystallographic rearran- gement, and sma& volume change during the

z=

d transformation.

One of the important properties of the thermoelastio transformation is its small driving force as oompared to that of the non-thermoelastic one / 8 / . It oan be expressed in terms of the heat effect and

the transformation temperature /9-II/. The latent heat of the 7-&transformation in Pe-24.5 Pt alloy is determined in one study /I2/. It gives the value of A H

P d for the two ultimate oon- ditions, disordered and annealed at 820 K for I20 hrs.

The present study is aimed at the calorimetric determination of the heat effects of the ~==dtransformation in the quenched Pe-24 Pt and Fe-25 Pt alloys and at the process of atomio ordering at 820 K.

The results obtained should be evaluated in connection with the processes occuring in alloys during annealing. The study also deals with the temperature dependence of deformation in order to establish the shape memory eff eot.

Experimental results.

-

The alloys were prepared by the method of le- vitatian meltina; in helium and cast into a cylindrical mould. This

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982455

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JOURNAL DE PHYSIQUE

was followed by a solution treatment at 1373 K for 24 hrs and by wa- ter quenching from I473 K. The ingots were cut into cylindrical spe- cimens 5x8 mm for calorimetry and wire specimens I mm in dm for the study of deformation. Capability of alloys to the shape memory effect was determined on wires under torsion deformation. The phase composition was determined by X-Ray analysis. The Fe-24 Pt and Fe-25 ?t alloys consist of two phases o(+[ after quenching, theref ore before calorimetric study they were cooled down in li'quid nitrogen in order to ascribe the heat effect to the full~~dtransformation. The heat effeots were measured in a DSC-111 Setaram calorimeter /3/. The error of the enthalpy determination was about 3%. The effect was separated from the eero line by the Berg method. The experiment was carried out as follows: the specimen was heated in the calorimeter up to 820 K, held at this temperature for some time and then cooled down to 150 K, Thereby one cycle included annealing-cooling-heating. A direct

transformation proceeds rapidly, in time, being of the same order as the calorimeter constant ( 2,

-

^{30 s),} ^therefore the Ys and Mf

points were taken at the beginning and maximum of the heat effect respectively. The reverse d - - r transformation is delayed, its duration is more than

,

therefore the points As and Af are taken at the beginning and end of the peak respectively. Table I sums up the data on temperatures and heats of the p d transformation after several cycles of heating and cooling and short-time annealing at 820 K di- rectly in the calorimeter. Pig.1 shows the heating curves of the Fe-25 Pt alloy for several cycles. The heating curves exhibit at least two effects for the reverse d-.-

d/

transformation, Their valu- es reduce, as is also the case for those of the y-d transformatton.

The As points of the first effect (I) reduce with the increase of the annealing duration, point As of the second effect (11) does not change until the effect ceases after 6 hrs of annealing, The heat effeots of the transformation are individual and of sharp shape, oorres- ponding to the high rate of the K-rd transformation, After 11.5 hrs of annealing the heat effect of the d--r transformation is strong- ly smeared, it can hardly be separated from the eero line and preoise- ly measured. The heat effect of the &-A transformation is small (about 150 d/mole) and it is proportional to the driving force of the thermoelastic martensitic transformation of this alloy. In the Fe-24 Pt alloy the heat effects of the

4-8

transformation divide into two after 2 hrs of annealing at 820 K, while sharp reduction of the A H value of the

r,--o(

transformation also occurs in the first cycles at the constant points of transformation. After 5 hrs of annealing the heat effect I1 at 650 K disappears, while the first one (I) observed at lower temperatures is smeared. Fig.2 shows the time dependence of the temperatures and heats of the r*o( transformation. Fig.3 shows the temperature dependence of deformation for the Fe-24 Pt and Fe-25 Pt alloys after annealing at various temperatures and holding times. The quenched alloys do not exhibit any noticeable shape memory effect. The Fe-25 Pt alloy (Fig.3a), annealed for 4 hrs at 820 K, recovers the accumulated strain energy over a wide temperature range

,

the process being noticeably two-stage and incomplete. After annealing for 20 hrs this alloy (~ig,3b) does not undergo martsnsitic transformation down to 77 K. For Fe-24 Pt alloy (Fig.3o), annealed for 24 hrs, complete recovery is observed on heating over the range of 175- 300 K, After annealing for 48 hrs at 873 K this range reduoes down to 77-170 K, Degree of the long-range order in the Fe-24 Pt alloy is not high after heat-treatments described and is about 0,6 and 0,7 respectively.

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4 93 37 9

-

red

-

6 5 5 760 4,8 1793

/

^3,6 ¹³⁴⁵

650 7 5 2.0 747

7

^1.9 ⁷¹⁰

1 300 /smeared I ,4 522 I smeared Nr _neal-^{o f}^an-

ing a t 820 K ,

(hrs)

Ms ,K _Hlf,K As, K A f , K

cal/g ^J/mole cal/g ^J/mole

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JOURNAL DE PHYSIQUE

Fig.1. Heat e f f e c t s of 4h.e

r - - - d (a) and d--y (b) t r a n s f o r m a -

c$e

3

t i o n i n t h e

Fe3Pt a l l o y .

al&-+g-@o

430 cych

4

540 -730 613

J / " o ~

n !i 10 T-

^hour

F i g - 2 , Dependence of t a e h e a t s and t e m p e r a t u r e s of t h e

rz

o( t r a n s f o r m a t i - on on h o l d i n g t i m e o f t h e a n n e a l i n g a t 820 K i n t h e Fe3Pt a l l o y .

D i s c u s s i o n

x n a l y s i s of t h e c a l o r i - m e t r i c d a t a shows, t h a t r e - d u c t i o n of A H

7 2 4

depen- d i n g on t h e a n n e a l i n g t i m e (Fig.2a) a t 820 K s h o u l d be connected w i t h , a t l e a s t , two d i f f e r e n t p r o c e s s e s . Cu- r i n g t h e f i r s t a n n e a l i n g s t a g e s , when A H d e c r e a s e s r a p i d l y and t h e t e m p e r a t u r e of t h e t r a n s f o r m a t i o n start does n o t change (Fig.2b), one may assume, t h a t o n l y t h e amount of t h e phase t r a n s f o r m e d i s d e c r e a s e d . F u r t h e r f o r l o n g e r a n n e a l i n g

t i m e t h e change of t h e t h e r - modynamic p a r a m e t e r s of t r a n s f o r m a t i o n i s p o s s i b l e . T h i s i s confirmed by t h e change of t h e t r a n s f o r m a t i - on t e m p e r a t u r e (Fig. 2b). The dependence of H on a n n e a l - i n g t i m e ( F i g . 2 ~ ) may be ap- proximated by t h e e q u a t i o n as f o l l o w s :

-

A dependence of t h a t kind may a r i s e when t h e d e c r e a s e of t h e amount of phase t r a n - sformed i s due t o d i f f u s i o n

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0-1 elastio loading, 1-2 accumulation of the deformation on cooling,

2-3 unloading, 3-4 reoovery of the deformation on heating.

a) Pe-25 Pt alloy annealed for 4 hrs at 820 K;

b) Fe-25 Pt alloy annealed for 20 hrs at 820 K;

c) Fe-24 Pt alloy annealed for 20 hrs at 820 K;

d l Fe-24 Pt allos ahnealed for 48 hrs at 870 K.

processes. X-Hay data showed also the

O( -phase in the alloys, annealed at 820 K together with partially ordered austenite. This con- firms the eutectoid decomposition: ?f- 0( +

+

Pe Pt above 820 K, therefore the amount of the -phase decreases.

When annealing the quenched alloys, whose concentration of thermal 3 vacancies is high, the initial rate of eutectoid -phase deoompo- sition is also high, then on cooling the decomposit on is interrupted by the martensitic transformation and flnally freezed down. Further on annealing, an atomio ordering is prevailing, effecting the martensitic transformation, which follows, Decrease of the A H

-

^value

of the

rzo(

transformation is slowing down. After annealing for 10 hrs the A H of the Fe-24 Pt alloy decreased down to 522 J/mole;

this is olose to the value obtained in /I3/ (347 J/mole) for the alloy, annealed for 120 hrs.

During the isothermal annealing at 820 K a heterogeneous prooess of atomio ordering takes place. Ordered and disordered austenite are simultaneously present in the alloy. An amount of the ordered phase increases, but the heat effeot of its martensitic transformation decreases. That is why up to the fifth cycle fJ H

d-z

^C1)- ^trans-

formation slightly changes in time. The presence of two maxima (I and 11) in the thermal effect of the transformation, with constant temperatures of one of them (XI) and with decreasing temperatures for

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C4-382 JOURNAL DE PHYSIQUE

the order (I), corresponds to the double bend in the curves of the temperature dependence of the electrical resistivity when heating the Fe3Pt alloy, as in /I/. These phenomena are, apparently, of the same origin.

Finally, one should bear in mind that partially ordered alloys exhibit practically complete shape memory effect. This is in accor- dance with the assumption /3/ that high degree of the long-range order XI2 is not a determinant criterion for shape memory. The idea about the inherited nuclei of martensite in austenite at the temperatures above As is not a speculatlve one for the Ee3Pt alloy, These may be the local regions of the

d

-phase, produced by the eutectoid reaction.

Thus, isothermal annealing at 820 K, followed by heating and cooling over the interval of the rzd transformation in the alloys close to the Fe Pt composition, results In the first cycles together with the atomic ordering A1 -L12 also in the eutectoid decompositi- 3 on o(

+

Fe Pt* Decrease of the

a

-phase amount causes sharp reduction of the haats of the transformation. On further cycles the martensitic transformation is mainly influenced by heterogeneous atomic ordering. Thereby, the transformatlon temperatures of the 8 4 - transformation also decrease, One should also take into account the rival phase transformations, sinoe they may exert an effect on both the formation of the nucleus and on the character of the d martensitic transformation as well as on the final properties of the martensite,

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Dunne D.P. and Wayman C.M. : Met. Trans. 4(1973), I37*

Kajiwara S, and Owen W,S,: Met, Trans, 5- 1974

,

^2047,

Tadaki T. and Shlmieu K.: Scripta Met.

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A.S.Tailashev, M.M,Matveeva, L. L.Ganzina. Alloys of the Rare and Refractory Metals with Special Physical Properties.

Moscow, Nauka, (1979) 304,

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